Latent heat recovery type water heater

ABSTRACT

A latent heat recovery type water heater A being designed such that water flown in a secondary heat exchanger  2  is heated, and the heated water is supplied to a primary heat exchanger  1  and is further heated. A controller  4  compares heat exchange efficiency η of the secondary heat exchanger  2  with a predetermined threshold value Th, and the controller  4  determines whether the primary heat exchanger  1  is in a normal condition or not. The threshold value Th is changed corresponding to temperature of inflow water to the secondary heat exchanger  2.  Accordingly, an abnormality such as scale adherence in the primary heat exchanger  1  is accurately detected regardless of the fact that the primary heat exchanger  1  and the secondary heat exchanger  2  are connected by a bypass path or not.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a latent heat recovery type water heater, more particularly to a latent heat recovery type water heater capable of detecting an abnormality such as scale adherence or exhaust clogging.

2. Description of the Related Art

One example of a latent heat recovery type water heater is disclosed in Patent Literature 1.

-   In the water heater disclosed in Patent Literature 1, sensible heat     and latent heat are subsequently recovered from the combustion gas     generated by a burner using a primary heat exchanger and a secondary     heat exchanger. Water to be heated passes through the secondary heat     exchanger and then is supplied to the primary heat exchanger. It is     for avoiding such disadvantage that the recovery efficiency of     latent heat decreases when the temperature of the inflow water to     the secondary heat exchanger rises.

When the above-mentioned water heater is used for a relatively long time, scale such as calcium carbonate may adhere to the inside of heat transfer tubes of the primary heat exchanger. Scale adherence easily appears when water is supplied in the heat transfer tube; however, scale adherence hardly appears in heat transfer tubes of the secondary heat exchanger because of low temperature in the secondary heat exchanger. Scale adherence may deteriorate the heat exchange efficiency of the primary heat exchanger and furthermore the water heater may have troubles when such adherence phenomenon becomes prominent. Therefore, it is desired to appropriately detect a phenomenon of scale adherence.

When the heat exchange efficiency of the primary heat exchanger is deteriorated because of scale adherence, the heat recovery amount by the secondary heat exchanger increases and compensates the deteriorated efficiency, so that the total heat recovery amount of the water heater is not largely deteriorated. Therefore, detection of the scale adherence in the primary heat exchanger becomes difficult.

In Patent Literature 1, for detecting scale adherence, each heat exchange efficiency of the primary heat exchanger and the secondary heat exchanger is calculated and the ratio is obtained. When the primary heat exchanger has scale adherence, the heat exchange efficiency of the secondary heat exchanger relatively increases. Therefore, scale adherence is able to be detected in such a manner.

There is still a room for improvement in the conventional art (the above-mentioned means disclosed in Patent Literature 1) as mentioned below.

The ratio of the heat exchange efficiency of the primary heat exchanger and that of the secondary heat exchanger does not change depending only on existence of scale adherence. It also changes depending on other factors such as conditions of the temperature of the inflow water to the secondary heat exchanger. When the temperature of the inflow water to the secondary heat exchanger becomes low, the relative heat exchange efficiency ratio of the secondary heat exchanger becomes high. Therefore, in the conventional art, it is difficult to improve detection accuracy of scale adherence and there is a room for improvement.

In the conventional art, it is required that all of the water having passed through the secondary heat exchanger is supplied to the primary heat exchanger. Therefore, a means corresponding to the bypass path 54 in FIG. 1, to be mentioned later, is provided. In addition, it is difficult to accurately detect scale adherence in the conventional art when only a part of water having passed through the secondary heat exchanger is supplied to the primary heat exchanger.

The above-mentioned explanation exemplifies scale adherence shown in the primary heat exchanger. However, there is a fear that the primary heat exchanger may have exhaust clogging (fin clogging). Exhaust clogging is caused when soot in combustion gas adheres to a plurality of fins provided with the heat transfer tube of the primary heat exchanger and accumulates thereon, thereby causing breakdown. The secondary heat exchanger receives the combustion gas having passed through the primary heat exchanger, thereby hardly causing exhaust clogging. In the conventional art, it is difficult to exactly detect exhaust clogging of the primary heat exchanger at high accuracy like the case of scale adherence.

CITATION LIST

Patent Literature 1: Japanese unexamined patent publication 2009-264684

SUMMARY OF THE INVENTION

An object of the present invention is to provide a latent heat recovery type water heater capable of resolving or reducing the above-mentioned disadvantages.

The present invention proposes the following technical means for solving the above-mentioned problems.

The latent heat recovery type water heater of the present invention comprises a primary heat exchanger having at least one heat transfer tube for recovering sensible heat from combustion gas generated by a burner, a secondary heat exchanger having at least one heat transfer tube for recovering latent heat from combustion gas of which heat is recovered by the primary heat exchanger, and a controller capable of calculating heat exchange efficiency of the secondary heat exchanger. Water entered in the secondary heat exchanger is heated, and the heated water is supplied to the primary heat exchanger and is further heated therein. When the controller calculates the heat exchange efficiency of the secondary heat exchanger, the controller compares the calculated heat exchange efficiency with a predetermined threshold value. In case that the calculated heat exchange efficiency is larger than the threshold value, the controller determines there is an abnormality. In case that the calculated heat exchange efficiency is not larger than the threshold value, the controller determines there is no abnormality. The threshold value is designed to be changed corresponding to the temperature of the inflow water to the secondary heat exchanger.

Preferably, the latent heat recovery type water heater of the present invention further comprises a bypass path for introducing a part of water having passed through the secondary heat exchanger to an outflow side path of the primary heat exchanger without supplying the water to the primary heat exchanger. Water having passed through the bypass path and water having passed through the primary heat exchanger are mixed in the outflow side path of the primary heat exchanger.

Preferably, the controller has a memory portion which stores data relating to the threshold value corresponding to the temperature of the inflow water to the secondary heat exchanger, and the threshold value to be compared with the heat exchange efficiency of the secondary heat exchanger is determined based on the data.

Preferably, the threshold value is larger than the heat exchange efficiency of the secondary heat exchanger when the primary heat exchanger is in a normal condition and the threshold value becomes small when the temperature of the inflow water to the secondary heat exchanger becomes high.

Other features and advantages will be apparent in the following detailed description of the preferred embodiments referring to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view diagrammatically explaining a latent heat recovery type water heater of the present invention.

FIG. 2 is a view diagrammatically showing one example of data of a threshold value stored in a controller provided with the latent heat recovery type water heater shown in FIG. 1.

FIG. 3 is a flow chart showing one example of the operation procedure of a controller provided with the latent heat recovery type water heater shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is explained in detail referring to the drawings.

A latent heat recovery type water heater A shown in FIG. 1 has a burner 3, a primary heat exchanger 1 for recovering sensible heat from the combustion gas generated by the burner 3, a secondary heat exchanger 2 for recovering latent heat from the combustion gas which has passed through the primary heat exchanger 1, and a controller 4. The burner 3 is a gas burner for burning combustion gas.

The primary heat exchanger 1 and the secondary heat exchanger 2 have heat transfer tubes 10 and 20, respectively. Hot water or cold water, referred to as water hereinafter, flows in the heat transfer tubes 10, 20 and the outer surfaces of the heat transfer tubes 10, 20 are exposed to combustion gas. The heat transfer tubes 10, 20 are designed to have a plurality of fins 10 a, 20 a on the outer circumferences for improving heat transfer efficiency. Water supplied to a water inlet 50 enters the secondary heat exchanger 2 via a flow path 51 and is heated. The heated water is supplied to the primary heat exchanger 1 through a flow path 52. The flow path 52 is connected to a flow path 53 on a water outlet side of the primary heat exchanger 1 via a bypass path 54. Not all of the water having passed through the secondary heat exchanger 2 is designed to be supplied to the primary heat exchanger 1. A part of the water is supplied to the primary heat exchanger 1, on the other hand, other part of the water is directly introduced into the flow path 53 via the bypass path 54. In such a configuration, the primary heat exchanger 1 is raised to high temperature because the amount of water in the primary heat exchanger 1 reduces, thereby improving condensation prevention function of the primary heat exchanger 1. Particularly, condensation of the primary heat exchanger 1 refers to a phenomenon in which moisture in combustion gas condensates and strongly acidic condensate water is generated in the primary heat exchanger 1. The strongly acidic condensate water becomes a factor for corroding the primary heat exchanger 1. The condensate water as mentioned above is not generated or is reduced by raising the primary heat exchanger 1 to high temperature, thereby enabling to prevent corrosion of the primary heat exchanger 1. The water having passed through the primary heat exchanger 1 and the water having passed through the bypass path 54 are mixed in the flow path 53 and the mixed water is supplied to a desired destination of water from a water outlet 55. The flow paths 51, 52 have temperature sensors Sa, Sb for detecting the temperature of inflow water and the temperature of the outflow water from the secondary heat exchanger 2 and a flow rate sensor Sc for detecting the inflow amount of water.

A controller 4, constituted with a microcomputer, is capable of operating and controlling each member of the latent heat recovery type water heater A. In addition, the controller 4 determines whether the primary heat exchanger 1 has an abnormality such as scale adherence or exhaust clogging. The detailed operations are explained later. A memory portion 40 of the controller 4 stores data D1 of a threshold value Th as shown in FIG. 2 as data for the above-mentioned determination.

Data D0 shown with a dotted line in FIG. 2 indicate the relation of the inflow water temperature and the heat exchange efficiency of the secondary heat exchanger 2 when the primary heat exchanger 1 is normal. “The primary heat exchanger 1 is normal” means the primary heat exchanger 1 operates in a normal way and does not cause scale adherence or exhaust clogging. The data D1 of the threshold value Th are the above-mentioned heat exchange efficiency value added with a predetermined value α, for example 3%, and are set for various inflow temperature of water.

Next, function of the above-mentioned latent heat type water heater is explained. In addition, one example of the operation procedure of the controller 4 is explained referring to the flow chart in FIG. 3.

When water is supplied to the secondary heat exchanger 2 and to the primary heat exchanger 1 and the burner 3 starts combustion drive, the controller 4 executes arithmetic operation of the heat exchange efficiency η of the secondary heat exchanger 2 (S1). The heat exchange efficiency η is obtained by the following formula 1.

η=Q1/Q2  formula 1

-   Q1 is the heated amount of water by the secondary heat exchanger 2     and is obtained by the formula Q1 =(T2-T1)·W, wherein T1 is the     inflow water temperature, T2 is the outflow water temperature from     the secondary heat exchanger 2, and W is the inflow amount of water.     These values are detected by the temperature sensors Sa, Sb and the     flow rate sensor Sc. -   Q2 is the combustion heat amount of the burner 3 and is obtained by     multiplying the consumed amount of fuel gas of the burner 3 and the     heat generation amount of the fuel gas, for example.

Next, the controller 4 selects data of the threshold value Th corresponding to the inflow water temperature T1 from the data D1 shown in FIG. 2 and compares the threshold value Th with the above-mentioned heat exchange efficiency η (S2, S3). When the formula η>Th is established by the comparison and is kept for a predetermined time, an abnormality such as scale adherence or exhaust clogging in the primary heat exchanger 1 is detected (S3:YES, S4:YES, S5). In such a case, alarm informing abnormality is operated or the burner 3 stops operation, which are not shown in FIG. 3. On the other hand, when the relation η>Th is not established, an abnormality is not detected (S3:NO, S6). A series of operations as mentioned above is continuously repeated while the latent heat recovery type water heater A is driven.

When the primary heat exchanger 1 has scale adherence or exhaust clogging and such phenomenon becomes apparent, the heat exchange efficiency of the primary heat exchanger 1 reduces, on the other hand the heat exchange efficiency η of the secondary heat exchanger 2 increases. In the preferred embodiment of the present invention, in view of the phenomenon in which the heat exchange efficiency η of the secondary heat exchanger 2 increases, the controller 4 determines there is an abnormality such as scale adherence or exhaust clogging when the heat exchange efficiency η becomes high enough to exceed the threshold value Th. On the other hand, considering that the heat exchange efficiency η is susceptible to the effect of the inflow water temperature, the threshold value Th is changed depending on the inflow water temperature. Therefore, in the preferred embodiment of the present invention, an abnormality such as scale adherence or exhaust clogging is able to be detected at high accuracy comparing with a conventional technology.

In addition, in the preferred embodiment of the present invention, the heat exchange efficiency of the primary heat exchanger 1 is not required to be considered, unlike the conventional art. Therefore, even when the amount of water supplied to the primary heat exchanger 1 is different from that supplied to the secondary heat exchanger 2 by the bypass path 54, an abnormality such as scale adherence or exhaust clogging is able to be precisely detected.

The controller 4 has the memory portion 40 storing data showing the relation of the temperature of the inflow water to the secondary heat exchanger 2 and the threshold value Th as mentioned above. The threshold value Th, an object to be compared with the heat exchange efficiency η of the secondary heat exchanger 2, is decided based on the data stored in the memory portion 40. Therefore, determination of the threshold value Th is able to be executed rapidly and appropriately.

The present invention is not limited to the above-mentioned preferred embodiment. The specific configuration of the members of the latent heat recovery type water heater of the present invention is freely designed within the intended scope of the present invention.

In the present invention, the bypass path 54 in the above-mentioned preferred embodiment may not be provided. The shape, material and number of the heat transfer tube are not limited as long as the primary heat exchanger and the secondary heat exchanger are designed to be able to recover heat using the heat transfer tubes. The burner may be an oil burner instead of the gas burner. 

1. A latent heat recovery type water heater, comprising: a primary heat exchanger having at least one heat transfer tube for recovering sensible heat from combustion gas generated by a burner; a secondary heat exchanger having at least one heat transfer tube for recovering latent heat from combustion gas of which heat is recovered by the primary heat exchanger; and a controller capable of calculating heat exchange efficiency of the secondary heat exchanger, wherein water entered in the secondary heat exchanger is heated, and the heated water is supplied to the primary heat exchanger and is further heated therein, and when the controller calculates the heat exchange efficiency of the secondary heat exchanger, the controller compares the calculated heat exchange efficiency with a predetermined threshold value, in case that the calculated heat exchange efficiency is larger than the threshold value, the controller determines there is an abnormality, in case that the calculated heat exchange efficiency is not larger than the threshold value, the controller determines there is no abnormality, the threshold value being changed corresponding to temperature of inflow water to the secondary heat exchanger.
 2. The latent heat recovery type water heater as set forth in claim 1, further comprising: a bypass path for introducing a part of water having passed through the secondary heat exchanger to an outflow side path of the primary heat exchanger without supplying the water to the primary heat exchanger, wherein: water having passed through the bypass path and water having passed through the primary heat exchanger are mixed in the outflow side path of the primary heat exchanger.
 3. The latent heat recovery type water heater as set forth in claim 1, wherein the controller has a memory portion which stores data relating to the threshold value corresponding to the temperature of inflow water to the secondary heat exchanger, and the threshold value to be compared with the heat exchange efficiency of the secondary heat exchanger is determined based on the data.
 4. The latent heat recovery type water heater as set forth in claim 1, wherein the threshold value is larger than the heat exchange efficiency of the secondary heat exchanger when the primary heat exchanger is in a normal condition and the threshold value becomes small when the temperature of inflow water to the secondary heat exchanger becomes high. 